As they execute, programs (C and C++ programs, in particular) should be deallocating all of the memory they allocate as they actually run.
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Later, upon deallocation, the deallocating routine will check, whether the guard bytes are modified and reports an error if appropriate.
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Objective-C uses ARC (Automatic Reference Counting) for deallocating unused objects. In Objective-J, objects are automatically deallocated by JavaScript's Garbage Collector.
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Circular dependencies may also cause memory leaks by preventing certain very primitive automatic garbage collectors (those that use reference counting) from deallocating unused objects.
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A program must allocate space in memory for the variables its functions use. Most programming languages use a call stack for storing the variables needed because it allows for fast and simple allocating and automatic deallocation of memory. Other programming languages use a heap for this, which allows for flexibility at a higher cost for allocating and deallocating memory. Both of these implementations have benefits and drawbacks in the context of continuations.
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Alternatively, when a region is deallocated, its list of blocks can be appended to a global freelist from which other regions may later allocate new blocks. With this simple scheme, it is not possible to deallocate individual objects in regions. The overall cost per allocated byte of this scheme is very low; almost all allocations involve only a comparison and an update to the next-free-position pointer. Deallocating a region is a constant-time operation, and is done rarely.
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Systems using regions may experience issues where regions become very large before they are deallocated and contain a large proportion of dead data; these are commonly called "leaks" (even though they are eventually freed). Eliminating leaks may involve restructuring the program, typically by introducing new, shorter-lifetime regions. Debugging this type of problem is especially difficult in systems using region inference, where the programmer must understand the underlying inference algorithm, or examine the verbose intermediate representation, to diagnose the issue. Tracing garbage collectors are more effective at deallocating this type of data in a timely manner without program changes; this was one justification for hybrid region/GC systems.
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Garbage consumes heap memory, and thus one wishes to collect it (to minimize memory use, allow faster memory allocation, and prevent out-of-memory errors by reducing heap fragmentation and memory use). However, collecting garbage takes time and, if done manually, requires coding overhead. Further, collecting garbage destroys objects and thus can cause calls to finalizers, executing potentially arbitrary code at an arbitrary point in the program's execution. Incorrect garbage collection (deallocating memory that is not garbage), primarily due to errors in manual garbage collection (rather than errors in garbage collectors), results in memory safety violations (that often create security holes) due to use of dangling pointers.
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When memory is nearly full, the MCP examines the working set, trying compaction (since the system is segmented, not paged), deallocating read-only segments (such as code-segments which can be restored from their original copy) and, as a last resort, rolling dirty data segments out to disk. Another way the B5000 provides a function of a MMU is in protection. Since all accesses are via the descriptor, the hardware can check that all accesses are within bounds and, in the case of a write, that the process has write permission. The MCP system is inherently secure and thus has no need of an MMU to provide this level of memory protection.
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It is generally the case that after an object is used, it is removed from memory to make room for other programs or objects to take that object's place. However, if there is sufficient memory or a program has a short run time, object destruction may not occur, memory simply being deallocated at process termination. In some cases object destruction simply consists of deallocating the memory, particularly in garbage-collected languages, or if the "object" is actually a plain old data structure. In other cases some work is performed prior to deallocation, particularly destroying member objects (in manual memory management), or deleting references from the object to other objects to decrement reference counts (in reference counting).
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Zeroing weak references is a feature in Objective-C ARC that automatically clears (sets to `nil`) weak-reference local variables, instance variables, and declared properties immediately before the object being pointed to starts deallocating. This ensures that the pointer goes to either a valid object or `nil`, and avoids dangling pointers. Prior to the introduction of this feature, "weak references" referred to references that were not retaining, but were not set to `nil` when the object they pointed to was deallocated (equivalent to `unsafe_unretained` in ARC), thus possibly leading to a dangling pointer. The programmer typically had to ensure that all possible weak references to an object were set to nil manually when it was being deallocated.
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Some statements were direct equivalents to VDU codes, such as CLS for VDU 12. Some statements were less exact equivalents as they incorporated functionality specific to BASIC as well as calling the OS routines; for example the statement would set screen mode and adjust the BASIC system variable HIMEM according to the amount of memory the new mode left available for BASIC, while would set the screen mode only, without altering HIMEM. This allowed a programmer to allocate a block of memory from BASIC for example to load machine code routines into by lowering the value of HIMEM at the start of a program, and still be free to switch screen modes without deallocating it as a side effect. There is one operating system command to write a character, OSWRCH, which is responsible for all text and graphics.
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